In the gasification of coal or oil, the process produces a desired synthesis gas but also produces considerable particulate solids such as ash. To efficiently utilize the synthesis gas so produced it is necessary to remove the entrained solids from the gas. This separation is often accomplished by a process of filtration conducted at elevated temperature and/or elevated pressure.
It is known to use, in such filtration processes, porous filter elements comprising a plurality of filter tubes, also referred to as candles because of the shape thereof, arranged in a parallel relationship and supported by a support tube or tube sheet.
The construction of the filter tubes will depend to a large extent upon the conditions under which the filtration is to be conducted. Under moderate filtering conditions including a filtration temperature of about 150.degree. C. and a filtration pressure of about atmospheric, the filtration tubes are constructed from polyester, acrylic or glass. When streams of entrained solids in gases resulting from gasification of coal or oil are to be filtered, the filtration conditions are typically more vigorous, e.g., using temperatures as high as 900.degree. C. and pressures from 5 to 30 times atmospheric. Under these conditions the filter tubes are constructed from porous ceramic or even porous, sintered steel.
The particles of solid to be removed by filtration are from about 0.2 micron to about 20 microns in size and are present in the gas in quantities of up to 10 parts per thousand parts by weight of gas, or even more. As filtration progresses, the deposit of particulates on the filter tubes increases as evidenced by an increased pressure drop. Eventually, the filter elements must be cleaned as by shaking the tubes, reversing the flow of gas or by other conventional means.
The construction of an industrial filter assembly containing a number of individual filtering elements, e.g., elements comprising a plurality of filter tubes, poses a number of mechanical problems. Some means of properly, conveniently and economically assembling the potentially hundreds of filter tubes must be devised. Some method of introducing the solids-containing fluid to be filtered must be employed and the fluid must be evenly distributed across the individual filter tubes. Moreover, there must be some method of cleaning the particulates from the filter tubes and subsequently collecting the particulates and removing them from the filter assembly.
Conventional filter assemblies for filtering solids from particulate-carrying fluids, particularly particulate-carrying gas, under vigorous filtration conditions typically comprise a main filtration vessel equipped with fluid inlets and fluid and particulate outlets and a single tube sheet attached to the interior wall of the vessel. This tube sheet has a plurality of filter tubes attached thereto. As the need for increased filtration capacity increases, the tube sheet is enlarged in diameter to support more filter tubes. At some diameter, the tube sheet will sag in the center unless the tube sheet is made sufficiently thick to retard such sagging. Eventually, this enlargement and thickening becomes economically unattractive. To prevent sagging by the provision of center supports is not generally satisfactory because of differential expansion and contraction of the supported tube sheet at the elevated temperatures of the filtration.
In Tassicker et al, U.S. Pat. No. 4,525,184, there is described a filter vessel wherein a plurality of filter assemblies are contained within the vessel and supported in a spaced apart relationship by a single support tube within the vessel. This tube, in addition to supporting these filter assemblies, cooperates with the assemblies and the vessel's outlet means to serve as a discharge tube for directing the filtered gas to the gas outlet. However, such devices having centrally-supported filter assemblies have operating problems when constructed on a large scale and there is risk of re-entraining the particulate solids in the filtered gas.
It would be of advantage to have a filter assembly for removing particulates entrained in a gas or other fluid under vigorous conditions of filtration which can be constructed for large scale operation and which reduces the re-entrainment of the filtered particulates in the filtered gas.